Induction heating coils



June 14, 1966 J. MERRETT INDUCTION HEATING OOILS 2 Sheets-Sheet 1 FiledJuly 12, 1963 June 14, 1966 J. MERRETT 3,

INDUCTION HEATING COILS Filed July 12, 1965 2 Sheets-Sheet 2 Fig.4

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United States Patent O 3,256,417- INDUCTION HEATING COILS John Merrett,Gerrards Cross, England, assignorto The Hoover Company, North Canton,Ohio, a corporation of Ohio vFiled July 12, 1963, Ser. No. 294,470Claims priority, application Great Britain, Aug. 9, 1962, 30,615/ 62 4Claims. (Cl. 21910.79)

The present invention relates to induction heating and is concerned withthe construction of a coupling coil for inducing eddy currents in asurface such, for example, as the bottom wall of a pot, to heat it forcooking.

According to the present invention a coupling coil is wound so that inany section through the axis there are conductors at different distancesfrom the top surface of the coil and comprises a number of separatecurrent paths electrically connected in parallel, each current pathhaving parts of its length nearest to the top surface of the coil andparts farther from it.

In one form of the invention the coil is wound from a strip of braidformed from a number of insulated conductors. The individual insulatedconductors may be formed of anodized aluminum wire.

. In an alternative form of the invention a flat coil for heating a flatsurface may comprise two or more axially superposed layers eachincluding a number of sections one within the other, and each electricalpath may include a section of each layer. Preferably, all' the sectionsin anyone layer are of substantially equal impedance.

Conveniently the sections arewound from flat strip or tape of conductingmaterial in which case the Width of the tape is preferably notsubstantially greater than the skin depth at the frequency to beemployed.

For heating a pot up to say eight inches in diameter a simple couplingcoil wound in a single spiral-from fiat tape conductor might have athickness between /8 and inch in order to produce suitable heatingpower.-

When a pot is placed on such a coil energized at a frequency of say20,000 cycles per second, the current will be concentrated in the toplayer of the spiral and in the lower layer of the pot. The restrictionof current flow in the pot is an advantage as it increases its effectiveelectrical resistance and hence the power that can be dissipated in itwith a given current. On the other hand, the restriction of current flowin the spiral coil is a disadvantage as it considerably increases thepower dissipated in the coil and is liable to cause overheating of thecoil as well as reducing the efiiciency of generation of heat in the potThus the ratio 'of heating power developed in the pot to heating powerdeveloped in the coil is proportional to the square root of the ratio ofresistivity of the pot to resistivity of the coil, and as it isdifficult in practice to make these resistivities very different withoutintroducing costly complexities, it is difficult to ensure that muchmore than half the heating power is developed in the pot.

An object of the invention is to force the current to fiow uniformlythrough the depth of the spiral coil, with a considerable reduction inits effective resistance and a considerable increase of overallefliciency.

The invention may be performed in various ways but certain specificembodiments will be described by way of example with reference to theaccompanying drawings, in which:

FIGURE 1 is a top view of one form of coil;

FIGURE 2 is a side view of the coil shown in FIG- URE 1;

FIGURE 3 is an enlarged view indicating the braiding of the conductor ofFIGURE 2;

FIGURE 4 is a fragmentary view of a coil divided into sections;

Patented June 14, 1966 "ice FIGURE 5 is a diagram of a coil such as thatof FIG- URE 4 showing one way of connecting the sections electricallyand to which a top plate has been added to support pots to be heated,and

FIGURE 6 is a diagram similar to FIGURE 5 showing another way ofelectrically connecting the; sections.

In the arrangement shown in FIGURES 1 to 3 the coil 10 is of fiatannular form wound in a spiral from a strip of braid formed of anodizedaluminum wire having ends 11 and 12. The wire may, or example, be .010inch diameter, of which twenty-four groups, each of two wires, arebraided in a conventional fashion. That is to say, as indicateddiagrammatically in FIGURE 3, twelve groups of wires 13 are wound in acoarse pitch right hand helix, while the other twelve groups of wires 14are wound in a coarse pitch left hand helix, each group passingalternately above and below successive pairs of groups of the oppositehand. After forming, such a tubular braid is flattened and forms a stripof thickness about .045 inch and width about .150 inch. This is wound toformthe coil 10. The ends 11 and 12 of the coil are connected to a highfrequency generator 13 connected to a source of alternating current 14,and the generator supplies current at a frequency up to 20,000 cyclesper second. The top surface of the coil 10 is suitably covered by aplate 9 of stainless steel on which the pots to be heated are supported.

In the embodiment shown in FIGURE 4 the coil 15 is wound from solidconducting strip. The coil comprises a number of sections each of flatannular form wound from narrow conducting tape. The sections are in fourlayers identified as 1, 2, 3 and 4 and each layer com-- prises fourconcentric annular sections A, B, C and D. The sections are electricallyconnected in parallel in groups of four so as to provide four parallelcurrent paths. As indicated in FIGURE 5 the four groups comprise,respectively, sections A1, B2, C3 and D4 connected in series by leads16, sections A2, B3, C4 and D1 connected in series by conductors 17,sections A3, B4, C1 and D2 joined by conductors 18 and sections A4, B1,C2 and D3 connected by leads 19. Each of the sections is connected inparallel by leads 21 and 22 to a high frequency generator 23 connectedto a source of current 24. The generator 23 supplies current at afrequency up to 20,000 cycles per second. A top plate 25 of stainlesssteel for supporting the pots to be heated is shown in FIGURE 5 abovethe coil 15.

The radial width of each annular portion AB-C and D becomesprogressively smaller so that each portion will have approximately equalsurface areas for the pots to be heated and also provide substantiallyequal impedance in each section.

Each section is wound with tape or wire having a width or diameter lessthan the skin depth at the frequency employed. The skin depth is givenin centimeters in free space where p is the resistivity, u is thepermeability and f is the frequency. Alternatively the skin depth isgiven in inches by 6 skin depth would be considerably less than thevalue given by the above formula according to the method of winding thecoil.

The several sections in each layer have approximately the same impedancewhen coupled to the bottom of a pot placed upon the top plate 25, andthe total impedance of each current path will be the same, as the wholecoil is symmetrical. This condition is satisfied since each section hasthe same projected area of winding, assuming the coils are close wound.In these circumstances as all the current paths are connected inparallel across the current supply 24, the input current will divideequally. Provided that the total thickness of the stack of coils is nottoo great (less than about inch) so that all coils are satisfactorilycoupled to the pot, the overall power transfer efficiency can beincreased, possibly to something of the order of 60% or even 80%..

Similarly in the case of the braided Winding of FIG- URES 1 to 3 theanodized surface provides insulation between adjacent wires and eachWire meanders from a position closer to the surface than any other, to aposition farther from the surface than any other. Thus the shape andposition of each wire is practically similar to that of any other, sothat the inductances of all the current paths are equal, and equalcurrents will flow through them all.

In the coil 29 shown in FIGURE 6 each current path is afforded by turnsor sections lying between the same radial limits. The paths are affordedby a number of concentric coils, E, F, G and H, each having a section ora number of turns close to the surface of a pot to be heated and others6, 7 and 8 farther away but near enough to contribute useful heating.The layers 5, 6, 7 and 8 of the coil sections E, F, G and H areconnected respectively in series by leads 30, 31, 32 and 33. Thesections E, F, G and H are connected in parallel by conductors 34 and 35to a suitable high frequency generator 36 which is connected to a sourceof alternating current 37. The coil 29 is also supplied with a suitabletop plate 38 of stainless steel to support the different sized pots tobe heated.

The irnpedances of the paths are matched so that when all coil sectionsare covered by a pot on the plate 38 the currents in them will besubstantially equal, and if some only are covered by a pot the currentsin them will remain substantially unaltered although the current in theuncovered paths will fall to a negligible value.

While I have shown and described several embodiments of my invention, itis to be understood that those embodiments are to be taken asillustrative only and not in a limiting sense. I do not wish to belimited to the particular structure shown and described but to includeall equivalent variations except as limited by the scope of the claims.

I claim:

1. A flat coupling coil suitable for inducing eddy currents in a flatsurface such as the metal bottom of a pot to heat it, in which the coilis wound with conductors so that in any section through the axis of thecoil the conductors are arranged at different distances from the surfacebeing heated, and including a number of separate current pathelectrically connected in parallel, each path having parts of its lengthnearest tothe surface being heated and parts farther from it, and saidcoil comprising a plurality of axially superposed layers each layerincluding a plurality of conductor sections one within the other andeach current path including a section of each layen.

2. A coil as described in claim 1 in which each layer includes a sectionin each of a number of annular zones, and all the sections of each suchzone are included in the same current path.

3. A coil as described in claim 1 in which all the sections in any onelayer are of substantially equal impedance.

4. A coil as described in calim 1 in which the sections are wound fromflat strip or tape of conducting material and the width of the tape isnot substantially greater than the skin depth at the frequency to beemployed.

References Cited by the Examiner UNITED STATES PATENTS 1,342,209 6/1920Giblin 336-490 1,698,650 1/1929 Parks 336 FOREIGN PATENTS 826,849 4/1938France. 507,556 9/1930 Germany.

12,923 5/1911 Great Britain.

ANTHONY BARTIS, Acting Primary Examiner.

RICHARD M. WOOD, Examiner.

BENDER, Assistant Examiner,

1. A FLAT COUPLING COIL SUITABLE FOR INDUCING EDDY CURRENTS IN A FLATSURFACE SUCH AS THE METAL BOTTOM OF A POT TO HEAT IT, IN WHICH THE COILIS WOUND WITH CONDUCTORS SO THAT IN ANY SECTION THROUGH THE AXIS OF THECOIL THE CONDUCTORS ARE ARRANGED AT DIFFERNET DISTANCE FROM THE SURFACEBEING HEATED, AND INCLUDING A NUMBER OF SEPARATE CURRENT PATHELECTRICALLY CONNECTED IN PARALLEL, EACH PATH HAVING PARTS OF ITS LENGTHNEAREST TO THE SURFACE BEING HEATED AND PARTS FARTHER FROM IT, AND SAIDCOIL COMPRISING A PLURALITY OF AXIALLY SUPERPOSED LAYERS EACH LAYERINCLUDING A PLURALITY OF CONDUCTOR SECTIONS ONE WITHIN THE OTHER ANDEACH CURRENT PATH INCLUDING A SECTION OF EACH LAYER.